At every step only one letter may be changed.
Once changed you have to keep that letter.
All letters from the previous word must occupy a new position, in effect we create an anagrammed proper word of those letters but with one replaced.

S P A NP A N TT R A PR A T ET I E R

Note that, in the above, A and T each occupy the same positions in more than one word.

New rule!
No letter may ever occupy a position it has already occupied.